The present invention relates to detecting a desired physiological parameter from physiological electrical signals, and more particularly to removing artifacts from the physiological signal in order to more accurately detect the desired parameter.
It is often required to detect a physiological parameter from electrical physiological signals. For example, parameters related to the functioning of the heart are detected using known electrocardiogram systems; parameters related to the functioning of the brain are detected using known electroencephalogram systems, and the parameters of blood oxygen concentration and pulse rate are detected using known oximeter systems. In the remainder of this application, pulse oximeter systems will be described in more detail, though one skilled in the art will understand that the systems, circuits, and methods in the description may be modified to apply to other systems which detect physiological parameters from electrical physiological signals.
Known oximeter systems use light signals to detect the blood oxygen concentration. Light of two different wavelengths is made incident on blood perfused flesh, and either transmitted or reflected light is detected, converted to an electrical physiological signal, and that electrical signal processed to detect the physiological parameter of blood oxygen concentration, all in a known manner. It is well known that many factors introduce noise into the electrical signal: for example patient movement, changes in ambient light level, and, to a lesser extent, EMI from power wiring and/or other operating electrical equipment in the vicinity of the oximeter system.
In order to enhance accuracy, much work has been done to detect accurately the electrical oximetry signals in the noise inherent in such systems. Some prior art systems transform the time domain electrical physiological signal (including noise) into the frequency domain, and perform further processing in the frequency domain. Such systems use a Fourier transform to transform the electrical physiological signal into the frequency domain. More specifically, a discrete Fourier transform of some form, preferably a fast Fourier transform (FFT) is used. The frequency domain signals are then analyzed to separate the physiological signal from the noise and detect the desired parameter.
U.S. Pat. No. 6,122,535, issued Sep. 19, 2000 to Kaestle et al., illustrates a system in which FFTs are calculated of both the IR and red light representative signals generated by the oxymetric sensor. From the FFTs a magnitude transform is calculated for both the red and IR signals. The magnitude transform for one of the signals is plotted on the x axis against the magnitude transform for the other of the two signals on the y axis. The resulting x-y plot includes what are termed needles extending radially away from the origin. The magnitude and angle of these needles, and other parameters related to them, are compiled in a table. Each entry in the table is scored according to various criteria. The entry with the highest score is selected as representing the actual pulse rate of the patient. Data from the FFT related to this entry is then processed to generate pulse rate and blood oxygen concentration (SpO2) parameters.
U.S. Pat. No. 6,094,592, issued Jul. 25, 2000 to Yorkey et al. illustrates another system in which FFTs are calculated on both the IR and red light representative signals generated by the oxmetric sensor. An SpO2 parameter is calculated for each and every point in the FFT. A histogram is then constructed of all the SpO2 parameters previously calculated. One of the SpO2 parameters is selected from the histogram information according to a set of rules. This SpO2 value is displayed as the blood oxygen concentration parameter.
U.S. Pat. No. 5,924,980, issued Jul. 20 1999 to Coetzee, illustrates yet another system in which FFTs are calculated on both the IR and red light representative signals generated by the oxymetric sensor. In this patent, xe2x80x9cgoodxe2x80x9d and xe2x80x9cbadxe2x80x9d portions of the light representative signals generated by the oxymetric sensor are identified by comparing the light representative signals to an xe2x80x9cidealxe2x80x9d waveform. Outliers are identified and deleted by correlating the red and IR light representative signals. As a result of the correlation the adverse effect of the noise signals is minimized, and the SpO2 value is calculated in a more accurate manner.
All of these prior art system require a substantial amount of processing, and consequently a substantial amount of power. A system which requires less processing and consumes less power is desirable.
In accordance with principles of the present invention, a system for detecting a physiological parameter from a physiological signal, includes a source of the physiological signal. Circuitry, coupled to the signal source, detects spectral peaks in the physiological signal. Calculating circuitry, coupled to the spectral peak detecting circuitry, calculates a parameter value corresponding to each detected spectral peak. Weighting circuitry coupled to the calculating circuitry and the spectral peak detecting circuit, assigns a weight to each peak according to a feature of a signal and the parameter value corresponding to that peak. Circuitry, coupled to the weighting circuitry, selects the peak according to a predetermined criterion. Output circuitry, coupled to the selecting circuitry and the calculating circuitry, then generates the parameter value corresponding to the selected peak.